End Members

Stoichiometry

In crustal metamorphic rocks, Si should be 3 cations, within
error. If it isn't, check the microprobe calibration.

In the garnets typical of metapelitic and metabasic rocks, Al is
the dominant trivalent cation. It should not exceed 2 cations per
formula unit. Most commonly it falls between 1.95 and 2.00, and the
deficit is presumed to be made up by ferric iron.

The divalent cations should total 3, though there may be a small
excess if ferric iron is present. The ideal cation total is 8. A
small excess may indicate ferric iron.

Fe recalculation, and problems regarding Fe-rich minerals

To recalculate ferric iron, normalise to 12 oxygens and 8 cations,
using the method of Droop (1987). The
ferric iron is usually assigned to the andradite end member, and it is
unusual for Fe3+ to exceed Ca.

You may already have noticed that garnet analyses performed on
many microprobe systems have rather high analytical totals, of 101%
or more. This appears to be a problem common to Fe-rich minerals (it
is sometimes seen in Fe-rich pyroxenes too), and it suggests that the
correction procedures for such minerals may not be entirely adequate.
The problem is not merely an overestimation of Fe, as the
stoichiometry is normally acceptable. Be suspicious about the
analysis quality, however, if you find that after ferric iron
recalculation, the total divalent cations are significantly less than
3.

Compositional zoning in garnets

Garnets commonly show compositional zoning, which results from
slow diffusion in the garnet lattice. Diffusion of the divalent
cations in garnet is negligible at low and medium grade, so that the
interior of a garnet is isolated from the rock matrix, but may become
extensive enough for garnet to reach a homogeneous equilibrium
composition at high grade. Two fundamentally differing styles of
zoning pattern can be distinguished.

Growth zoning, in which a growing garnet does not homogenise
the composition of its interior, but adds successive shells of
material whose composition reflects equilibria and processes in
the rock matrix. Such garnets tend to have Mn and/or Ca-rich
cores, with XMg increasing steadily towards the rim.
This is a fractionation process. This style is found in garnets
which have grown at temperatures below about 650°C.

Diffusional zoning, in which a pre-existing garnet is modified
in composition by exchange of material with the rock matrix, and
the compositional gradients are frozen in on cooling before a new
homogeneous composition can be achieved. The usual result is a
garnet with a depletion in Mg, and commonly also an enrichment in
Mn, at the rim of the garnet. This style is found in garnets which
have experienced temperatures above about 600°C.

The garnet above, from a lower-grade part of the High Himalayan
slab, shows prograde growth zoning.

This garnet, from the high-grade part (Sil zone) of the High
Himalayan Slab, shows retrograde diffusional zoning.